42 
Having obtained so much, and as it appeared by the 
turbid state of the water that the mercury was impure, the 
tube was emptied, washed out several times, both with 
water and a solution of nitrate of mercury, and was then 
refilled as before with water and carefully purified mercury. 
At first it presented much the same frosted appearance as 
before, and there was no suspension. 
With a view to expedite matters the board carrying the 
tube was taken off its pivot and laid fiat on a table nearly 
horizontal; in this position it was so adjusted that the water 
and mercury both extended all along the tube. The tube 
was then connected by an indiarubber tube with an air 
pump, and the air pumped off until, and so long as, the 
water boiled in the tube. 
The board was then turned over on its edge so that the 
water might come in contact with that part of the tube 
which had been previously below the mercury. 
Having been turned back into its first position and ad- 
justed so that the closed end and long limb were slightly 
lower than the rest, the pump was kept working, and the end 
of the board at which was the closed end of the tube was 
gently hit with the hand. At first this caused the mercury 
to chatter all along the tube, and wherever the mercury 
broke, a minute bubble of an air or steam showed itself ; these 
passed slowly along to the open end ; until after this had 
been continued for some time the chattering ceased and the 
last bubble had passed out. 
Keeping the closed end lowest, and without breaking the 
connection with the pump, the board was replaced on the 
pivot and the tube erected. The mercury remained sus- 
pended until the tube was nearly erect, and this without 
any assistance from the air on the open end, so that the 
tension was nearly 90 inches. 
The same process of tapping was then repeated, and the 
tube replaced and left with the closed end downwards and 
